Mobile devices can determine proximity to each other based on location. Each device can obtain a current location. Generally, to obtain a current location, a mobile device can use data based on three sources: 1) WiFi hotspots, 2) cellular towers, or 3) a Global Positioning System (GPS). In particular, the mobile device sends data from any of the three sources to a server, and the server responds with location coordinates, e.g., longitude and latitude coordinates. A mobile device can compare its location to other devices' locations, e.g., by measuring distance between locations, to determine proximity. For example, if the measured distances are lower than a predetermined threshold, the devices can be determined to be in proximity to each other.